Conventional methods for forming structures such as electronic circuits involve plating, lithography and etching steps. These methods are well suited for high-volume production. However, they involve many steps and much wasted material as exposed photoresist is etched away. In another approach, three-dimensional structures can be formed by depositing layer after layer of material using drop-on demand printing methods. Electronic circuit elements are an example of such structures, which can be formed by printing a number of discrete layers on a substrate using materials having specific electrical properties. For example, a transistor can be formed by printing conducting, semiconducting and insulating materials in a particular layered pattern.
Drop-on-demand printing is a known printing technique where a droplet of ink is ejected by a thermal or piezoelectric inkjet print head. The droplet is ejected onto a substrate where the droplet dries and forms a dot of a pattern (e.g., a printed photo). In contrast to etching procedures, there is no wasted material.
A three-dimensional structure can be formed by dispensing layers of materials according to the patterns determined by a three-dimensional digital representation. Crump in U.S. Pat. No. 5,121,329, issued on 9 Jun. 1992, describes a method of forming three-dimensional structures using a dispensing head connected to a CAD system. The dispensing head can dispense material at a controlled rate onto a substrate in a predetermined pattern dictated by the CAD system. Materials are heated above their solidification temperatures and dispensed as fluids, which then solidify after deposition and cooling. This method is limited to materials that can be solidified in this way.
The formation of three-dimensional structures by selectively irradiating liquid photo-curable polymers has also been described in U.S. Pat. No. 4,575,330 (issued to Hull on 11 Mar. 1986), and U.S. Pat. Nos. 4,752,498 and 4,801,477 (issued to Fudim on 21 Jun. 1988 and 31 Jan. 1989, respectively). This technique, which is known as photosolidication, involves focussing ultraviolet (UV) light in a predetermined pattern either over the surface of a layer of liquid or within the volume of a liquid to cure (solidify) polymer material. Although this method also enables the design of objects using a CAD package, it is limited in the way different materials can be incorporated in the object as the liquids mix before curing.
U.S. Pat. Nos. 6,503,831 and 6,713,389 (issued to Speakman on 7 Jan. 2003 and 30 Mar. 2004 respectively) describe drop-on-demand printing of inks for electronic circuit elements. Curing (or solidification) of printed material is achieved using conventional drying and/or radiation-enhanced drying or curing. The curing process can include radiation-induced cross-linking of organic materials. In particular, Speakman describes a radiation source close to the nozzle (on the print head) that can be used to treat deposited material either before, during or after deposition. One of the advantages of irradiating in-flight is to partially cure the material before deposition and thus reduce dot sizes before impact on the substrate. In general, the term “cure” with relation to polymer materials is used to refer to solidification of the deposited material.
Mogensen in U.S. Pat. No. 6,697,694 issued on 24 Feb. 2004 describes a similar method for printing flexible circuits by printing layers of materials using techniques that include drop-on-demand printing. In this patent, a method and apparatus is described whereby materials are dispensed on a flexible substrate in a predetermined pattern using a dispensing unit which can plot patterns using motions in the x, y and z axes relative to the substrate. Printed material is then cured by a separate curing unit, which can also be moved relative to the substrate. Layers are formed by successively printing and then curing each layer. The described curing unit can either provide UV, infrared, or gamma radiation. Alternatively, curing can be achieved using heating methods.
U.S. Patent Application No. 2004/0041892 (Yoneyama et al.) describes a method of tuning the power of the curing irradiation (used with polymer inks) depending on the humidity measured by a sensor located close to the print head. The irradiation power is controlled within the circuitry of the printer and is used simply to maximise the polymerisation of the deposited ink.
Drop-on-demand printing has also been used to deposit inorganic nanoparticle materials that can be cured to form conductive elements. In these cases, the curing process results in the nanoparticles sintering or fusing to form conductive elements which have a lower resistance. In particular, curing of metal nanoparticle films has been achieved by heating the printed inks to temperatures of 150 to 200° C. However, this heating step limits substrates that can be used to those that can survive the curing temperatures required. More recently, Chung et al. have described a method of sintering metal nanoparticle films by irradiating the films with an Argon ion laser (514 nm) in “In-tandem deposition and sintering of printed gold nanoparticle inks induced by continuous Gaussian laser irradiation” published in Applied Physics A, volume 79, 1259-1261 in 2004. Like heat curing, laser irradiation can cause coalescence of the individual nanoparticles resulting in conductive gold films. Curing of nanoparticle inks using white light irradiation (provided by flash lamps used by cameras) has also been described in the PCT Patent Publication No. WO 03/018645 (Reda et al.). These irradiation methods of curing are advantageous because the curing step does not necessarily damage the substrate thus allowing a wider range of substrates to be used (e.g., flexible plastics).